A speed reduction gear is used in an electric power steering apparatus of an automobile. For example, with a column type EPS, a rotation of an electric motor is reduced in speed and amplified in output by being transmitted from a small gear, such as a worm gear, to a large gear, such as a worm wheel, in the speed reduction gear and is thereafter imparted to a steering shaft to perform torque assist of a steering operation. A region of the speed reduction gear that includes at least a portion of engagement of the small gear and the large gear is filled with grease or other lubricant composition for lubrication.
In a generally used lubricant composition, for example, a thickener, such as a urea thickener, and an additive, such as a low friction additive, are added to a base oil, such as a synthetic hydrocarbon oil. As the low friction additive, a metal soap, such as lithium stearate and calcium stearate, is used. Among these, lithium stearate is especially excellent in function as a low friction additive, that is, in the function of lowering a friction coefficient of the lubricant composition and is thus used widely (see, for example, Patent Documents 1 and 2).
By using a lubricant composition that contains lithium stearate, an efficiency of speed reduction in a speed reduction gear can be improved and a torque of an electric power steering apparatus incorporating the speed reduction gear can be reduced to improve a steering sensation of an automobile incorporating the electric power steering apparatus. However, especially under a high temperature environment, such as in a periphery of an engine of an automobile, a conventional lubricant composition that contains lithium stearate expresses an attacking property against polyamide resin, etc., that is popularly used as a component material of mechanical parts in mechanical fields and may cause a problem of shortening a durability life of a mechanical part (for example, a gear) made of the polyamide resin, etc.
Zinc stearate is known as a metal soap, which, when contained in a lubricant composition, can function as a satisfactory low friction additive substantially equivalent to lithium stearate (see, for example, Patent Documents 3 to 5). Also, according to examination by the inventors, zinc stearate provides an action of alleviating the attacking property against polyamide resin, etc. Use of a lubricant composition containing zinc stearate in place of lithium stearate in combination with a gear, etc., made of polyamide resin, etc., can thus be considered.